In the refining of petroleum oils, numerous processes and treatments utilize water or aqueous solutions of chemicals which in turn produce numerous waste waters which must be disposed of. However, such waste waters often contain toxic pollutants which create health hazards if discharged to the surface of the earth and sewers, streams or other bodies of water. As a result, the U.S. Environmental Protection Agency (EPA) has identified a large number of such toxic pollutants and set maximum limits which a waste water may contain in order to be safely discharged to the surface and sewers, streams or other bodies of water. A partially troublesome material in this category is chromium. In order to control the discharge of chromium pollutants in waste waters, the EPA has set limits, in a neighborhood of 0.25 mg/l of total chromium and 0.005 mg/l of hexavalent chromium, which may be present in discharged waste waters. These limits create disposal problems in many industries but are particularly troublesome in petroleum refining operations. Chromium compounds are utilized in a variety of refinery treatments such as chromium-containing catalysts for various catalytic processes, such as chromic chloride for the polymerization of olefins, chromic oxide for organic synthesis, etc. To the extent that the products of such catalytic processes are treated with an aqueous solution or water, for purposes of purifying the product, an aqueous waste solution will be produced which often contains chromium in a water-soluble form. The most frequent occurrence of contaminating chromium in refinery waste waters results from the utilization of chromium containing materials as corrosion inhibitors in boiler feed waters, cooling tower waters and the like. As steam is produced in a boiler or water evaporates in a cooling tower, solid salts tend to become concentrated in the water, thereby requiring that a portion of the water be intervally or continuously removed. Chromium from the corrosion inhibitors also builds up in this "blowdown" water. The present invention is particularly applicable to such blowdown waters from the numerous cooling towers utilized in refinery operations. These cooling tower blowdown streams contain soluble chromium compounds significantly in excess of EPA standards and accordingly, the water cannot be simply discharged to the surface and a sewer, stream or other body of water. The several alternatives available are, in some fashion, to remove substantially all of the of the chromium at the plant site, transport the waste waters to a commercial treating facility or utilize less expensive nonchromium-containing corrosion inhibitors or more expensive chromium-containing corrosion inhibitors. In any event, all of these alternatives have heretofore added substantially to the expense of disposing of the waste water.
It is also known, in the petroleum refining art, that most petroleum oils and natural gases contain varying amounts of sulfur in the form of hydrogen sulfide, mercaptans and the like. Accordingly, it is necessary to remove the sulfur from crude oil fractions or products of various refining operations in order to produce the final refinery products for sale, since sulfur results in the formation of corrosive materials as well as air pollutants such as SO.sub.x pollutants. This problem has been exaggerated by the necessity of refining less desirable oils containing higher concentrations of sulfur. One inexpensive technique for removing hydrogen sulfide and lower molecule weight mercaptans from petroleum streams is the caustic washing of the stream. During such caustic washing the sulphur will generally be removed as sodium hydrosulfide or sodium sulfide, both of which are soluble in water. As a result, substantial volumes of used or "spent" caustic solutions are produced as aqueous wastes which also must be disposed of in some manner. In plant treatment to recover the caustic treating agent and/or sulfur and transportation to commercial waste water processors will generally not be economically feasible. As a result, a common practice in the art is to send these solutions to evaporation ponds. However, this solution obviously requires a great deal of pond space and there is always the possibilty that contaminants from the pond may seep into ground waters.
It would therefore be highly desirable, in refinery operations, to be able to effectively and economically treat waste waters to remove hazardous or undesirable components therefrom, particularly if one waste water could be utilized to treat another waste water.